Electrodeposition of zinc-copper alloys



United States Patent 2,730,492 ELECTRODEPOSITION OF ZINC-COPPER ALLOYS Allan E. Chester, Highland Park, Ill., assignor to Poor & Company, Chicago, Ill., a corporation of Delaware No Drawing. Application December 7, 1951,

' Serial No. 260,557

14 Claims. (Cl. 204-44) This invention relates to new and useful electroplating compositions and to a new and improved method for the electrodeposition of alloys of zinc and copper.

One of the objects of the invention is to provide a new and improved method for extending the range of current densities employed in electrodepositing white alloys of zinc and copper from cyanide-zinc-copper plating baths. Another object of the invention is to prepare new and useful cyanide-zinc-copper plating baths.

An additional object of the invention is to provide a new and improved method for electrodepositing bright zinc-copper alloy plates.

Still another object of the invention is to provide a method of electroplating alloys of zinc and copper whereby a bright zinc-copper alloy plate is obtained on polished steel which can be lacquered or chrome plated directly without buffing. Other objects will appear hereinafter.

In accordance with the invention it has been found that the range of current densities at which bright deposits of white alloys of zinc and copper can be obtained is extended by incorporating with the electrolyte from which such alloys are electrodeposited a quantity of a bath soluble dithiocarbamate, preferably zinc dimethyl dithiocarbamate or piperidine pentamethylene dithiocarbamate. Certain methods are known in the art for electrodepositing white alloys of zinc and copper but in order to obtain a bright plate it has heretofore been necessary to use current densities not exceeding about 20 amperes per square foot. One reason for this appears to be that the amount of copper electrodeposited onto the article to be plated increases as the current density increases. When the relative proportion of copper, as compared with that of zinc in the electrodeposited plate, exceeds a certain amount the desired result is not obtained.

In order to obtain a bright deposit of an alloy of zinc and copper which is suitable for chromium plating without bufiing it is desirable to employ at least four addi tion agents in the preparation of a cyanide zinc-copper electroplating bath, as follows:

1. A purifier, for example, sodium sulfide, sodium polysulfide, ammonium sulfide or ammonium polysulfides.

2. Zinc in the form of a zinc aldonate, preferably zinc gluconate.

3. A bath soluble dithiocarbamate, preferably zinc dimethyl dithiocarbamate.

4. A brightening agent, preferably the reaction product of an oxyaldehyde and an amine containing primary and/or secondary amino groups and a water solubilizing radical such as hydroxy, carboxy or sulfonic radicals or salts thereof.

The invention will be illustrated but is not limited by the following example in which the quantities are stated in parts by weight unless otherwise indicated.

Example An electrolyte is prepared by dissolving zinc cyanide, sodium hydroxide and sodium copper cyanide NaeCu- (ON); in water to produce a bath containing .99 ounce copper and zinc.

per gallon of Cu, 4.50 ounces per gallon of Zn, and adding enough sodium cyanide and sodium hydroxide to give a bath containing 13 ounces per gallon of total NaON and 7 ounces per gallon of NaOH. This bath has a pH of about 11.95.

To the above bath there is added 15 to 35 grams per gallon of zinc gluconate.

Approximately 3 cc. per gallon of a sodium poiysulfide solution are mixed with the resultant product which is then filtered through a filter cake made up of activated carbon and asbestos in approximately equal proportions.

To the filtrate there is added 0.5 to 6 grams per gallon of zinc dimethyl dithiocarbamate.

To the resultant product there is then added 5 to 30 cc. per gallon of a brightening agent, for example, the product obtained by reacting 10% anisic aldehyde, 5% heliotropin and of mixed isopropanol amines.

Finally the product is filtered through a filter cake made of asbestos or wood pulp only.

The zinc gluconate is prepared by heating a commercial 50% gluconic acid solution to about F. for a time sufficiently long to melt any crystalline lactone that might be present, then adding one mole of lead free zinc oxide for every two moles of gluconic acid present and heating the mixture to 170 F. with agitation. The temperature is held at 170 F. for 10 minutes or until solution of the ingredients is complete. The resultant product is cooled to 120 F. in the liquid phase and poured into large stoneware vessels and cooled until a Waxy solid forms along with some supernatant liquor. The liquor is mainly glucose and water and is removed from the solid by filtration. The residue is placed in drying trays where it may be Washed with ice water to remove traces of residual glucose. The product is then dried by heating at temperatures below 250 F. until it becomes dry and hard, at which time it is ground in a hammer mill or in some other suitable manner.

The sodium polysulfide solution employed in the foregoing example can be prepared by mixing 1200 pounds of sodium sulfide with 480 pounds of powdered sulfur and making up to 480 gallons with water.

The brightening agent used in the foregoing example is prepared by reacting the anisic aldehyde, heliotropin and the mixed isopropanol amines by beating them together in approximately equimolecular proportions of aldehyde to amine at moderate temperatures up to about F. A slight excess of the amine is preferably employed and the reaction will take place at room temperatures around 75 F. The mixed isopropanol amines consist essentially of 43% tertiary isopropanol amine, 43 secondary isopropanol amine and 14% primary isopropanol amine.

It will be apparent from a consideration of the foregoing example that the zinc is present in the electroplating bath as complex ions in the form of sodium zinc cyanide, sodium zincate, zinc dimethyl dithiocarbamate and Zinc gluconate. The copper is present as sodium copper cyanide Na2Cu(CN)3.

The zinc gluconate and the zinc dimethyl dithiocarbamate may be considered to be carriers, that is, they function to influence grain refinement in the electrodeposited metal and to extend the range of current densities at which the electrodeposition may be effected without burning the metal.

The anodes employed in electrodepositing an alloy of zinc and copper from a plating bath of the type previously described can be alloys of zinc and copper, for example, an alloy containing 82% zinc and 18% copper. Alternatively, a zinc anode can be used and the copper can be added to the bath chemically. If desired, an external dialysis cell can be used to supply the copper ions or both Because copper dissolves faster than mixtures of zinc and copper that less current is used in zinc it is also possible to use with split anode circuits so dissolving the copper.

Although the bath preferably contains 82 parts of zinc to 18 parts of copper, the range may be varied from about 15 to 30 parts of copper to 85 to 70 parts of zinc and still obtain a white alloy of zinc and copper.

During the operation of the electroplating process it is desirable to maintain a ratio between total cyanide and total metal to be electrodeposited within the range of 2.35 to 2.55, preferably about 2.46. This is usually accomplished by adding a sufiicient amount of cyanide to the bath to bring the ratio Within the aforesaid range.

The percent of copper deposited from the foregoing bath increases as the current density is increased, for example as follows:

Throwing Power Percent of Copper Eleetrorleposited Current Density Amperes per Square Foot at the relatively high current practiced. The essential feature of the invention is the discovery that the bath soluble dithiocarbamates, more particularly those containing the groups are very effective in making it possible to use higher current densities without burning the electrodeposited plate. For example, although the aldonic acid derivatives, such as Zinc gluconate, have an anti-burning effect and make it possible to operate at current densities as high as 25 amperes per square foot without the addition of the dithiocarbamate, with the addition of the dithiocarbamate the usable current density is extended to 80 amperes per square foot.

The number of commercially available dithiocarbamates is rather limited. In addition to those already mentioned there are zinc ethylene bisdithiocarbamate and disodium ethylene bisdithiocarbamate.

The brightening agent may be omitted from the bath where the article to be plated is to be subjected to a subsequent buifing operation. In general, however, it is preferable to employ a brightening agent in order to obtain a plated article that has sufficient brightness directly out of the bath to permit lacquering or the application of a chromium plate without the necessity of bufiing. Preferred brightening agents for the purpose of the invention are the condensation product of furfural and anthranilic acid, piperonal and anthranilic acid, anisic aldehyde and Z-methyl Z-amino l-propanol, anisic aldehyde and mixed isopropanol amines, mixtures of piperonal and anisic aldehyde condensed with mixed isopropanol amines, mixtures of anisic aldehyde and piperonal condensed with Z-methyl 2-amino l-propanol, furfural dispersed with gluconic acid, anisic aldehyde dispersed with gluconic acid, piperonal dispersed with gluconic acid, reaction products of gelatin, an aldonic acid and furfural, reaction products of protein rich seed meals, an aldonic acid and furfural, and other products of a similar nature. The aldehyde condensation products employed as brightening agents are generally characterized by solubility in alkaline aqueous solutions.

When relatively high current densities are used in electroplating White alloys of zinc and copper in accordance with this invention the plating bath described in the fore going example represents a preferred manner of practicing the invention. However, where it is desired to operate at lower current densities, as in barrel plating, other additives may be employed in the bath. Thus, additional carriers may be added such as, for example, the proteinaldonic acid reaction products described in U. S. Patent No. 2,458,504. Such protein carriers are reaction products of gelatin or vegetable protein rich seed meals with aldonic acids, e. g., gluconic, mannonic, galactonic and arabonic, or the lactones of these acids.

Although the invention is applicable primarily to the plating of predominantly zinc alloys of zinc and copper which have a silvery or whitish appearance it will be understood that the invention can be applied to the plating of brass generally.

The most important advantage of the invention is the provision of a method and a plating bath which make it possible to plate white alloys of zinc and copper at much higher current densities than heretofore used. As a result, a plate of a given thickness can be obtained in a much shorter period of time, thereby speeding up the entire plating operation and greatly increasing the capacity or output of the plating plant.

The employment of the reaction products of oxyaldehydes and amines containing primary and/or secondary amino groups and a water solubilizing group as brightening agents for cyanide zinc-copper electroplating baths is described and claimed in my copending application U. S. Serial No. 246,335, filed September 12, 1951.

The zinc gluconate prepared by the reaction of zinc oxide and gluconic acid is described and claimed as a new composition of matter in my application U. S. Serial No. 180,500, filed August 19, 1950, which is a division of my application Serial No. 34,127, filed June 19, 1948. which matured into U. S. Patent 2,576,997 on December 4, 1951.

The invention is hereby claimed as follows:

1. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution a bath soluble dithiocarbamate having a radical fromthe group consisting of S S S OH:

-S CH3 S H the quantity of said dithiocarbamate being sufficient to extend the usable range of current densities at which alloys of zinc and copper can be electrodeposited from said bath.

2. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution a zinc aldonate and a bath soluble dithiocarbamate having a radical from the group consisting of C-NHi and and CH3 CH -S/ oHs S/ \H the quantity of said dithiocarbamate being sufficient to extend the usable range of current densities at which alloys of zinc and copper can be electrodeposited from said bath.

3. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution a zinc gluconate and a bath soluble dithiocarbamate having a radical from the group consisting of and and C-N -N S CHs S/ \H the quantity of said dithiocarbamate being sufiicient to extend the usable range of current densities at which alloys of zinc and copper can be electrodeposited from said bath.

and and 4. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution a bath soluble dithiocarbamate having a radical from the group consisting of CNHa and c N s -s CH3 S H the weight ratio of zinc to copper in said bath being within the range of 85 to 70 parts of zinc to 15 to 30 parts of copper, the quantity of said dithiocarbamate being sufficient to extend the usable range of current densities at which a white alloy of zinc and copper can be electrodeposited from said bath.

5. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution a quantity of zinc dimethyl dithiocarbamate suflicient to extend the usable range of current densities at which an alloy of zinc and copper can be electrodeposited from said bath.

6. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution a quantity of piperidine pentamethylene dithiocarbamate sufficient to extend the usable range of current densities at which an alloy of zinc and copper can be electrodeposited from said bath.

7. A plating bath comprising an alkaline zinc copper cyanide bath containing in solution zinc gluconate, a bath soluble condensation product of an oxyaldehyde and an amine from the group consisting of primary and secondary amines effective as a brightening agent for said bath, and a quantity of a bath soluble dithiocarbamate containing a radical from the group consisting of S O NH: and

copper can be electrodeposited from said bath.

8. A plating bath consisting essentially of an alkaline aqueous zinc copper cyanide bath containing 15 to 35 grams per gallon of zinc gluconate, 0.5 to 6 grams per gallon of zinc dimethyl dithiocarbamate, to 30 cc. per gallon of the reaction product of 10% by weight anisic aldehyde, 5% by weight heliotropin and 85% by weight mixed isopropanol amines, a weight ratio of total cyanide to total plating metal within the range of 2.35 to 2.55 and a weight ratio of zinc to copper within the range from 85 to 70 parts of zinc to to 30 parts of copper. 9. A method of increasing the usable current density of an alkaline zinc copper cyanide bath which comprises dissolving in said bath a bath soluble dithiocarbamate containing a radical from the group consisting of S\ S\ CH3 S\ C NHJ and \CN/ and \CN/ alloys of zinc and copper can be electrodeposited from said bath.

10. A method of increasing the usable current density of an alkaline zinc copper cyanide bath which comprises dissolving in said bath zinc dimethyl dithiocarbamate in an amount sufficient to extend the usable range of current densities at which alloys of zinc and copper can be electrodeposited from said bath.

11. A method of increasing the usable current density of an alkaline zinc copper cyanide bath which comprises dissolving in said bath piperidine pentamethylene dithiocarbamate in an amount sufiicient to extend the usable range of current densities at which alloys of zinc and copper can be electrodeposited from said bath.

12. A method of electrodepositing a white alloy of zinc and copper which comprises electrodepositing said alloy from an alkaline zinc copper cyanide bath containing in solution zinc gluconate and a bath soluble dithiocarbamate having a radical from the group consisting of \C NH2 and CN and CN -s -s om S H the quantity of said dithiocarbamate being suflicient to extend the usable range of current densities at which alloys of zinc and copper can be electrodeposited from said bath.

13. A method of electrodepositing a white alloy of zinc and copper which comprises electrodepositing said alloy from an alkaline zinc copper cyanide bath containing in solution zinc gluconate, a bath soluble dithiocarbamate having a radical from the group consisting of S\ S\ /CH; S\ OH: O-NH: and O-N and 0N S/ -S/ GHa S/ \H and a reaction product of an oxyaldehyde and an amine containing amino groups from the group consisting of primary and secondary amino groups and a water solubilizing radical from the group consisting of hydroxy, carboxy and sulfonic radicals and salts thereof, the quantity of said dithiocarbamate being sufiicient to extend the usable range of current densities at which said alloy can be electrodeposited from said bath.

14. A method of electrodepositing a white alloy of zinc and copper which comprises electrodepositing said alloy from an alkaline zinc copper cyanide bath containing in solution zinc gluconate and zinc dimethyl dithiocarbamate, the quantity of said dithiocarbamate being sufiicient to extend the usable range of current densities at which said alloy can be electrodeposited from said bath.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PLATING BATH COMPRISING AN ALKALINE ZINC COPPER CYANIDE BATH CONTAINING IN SOLUTION A BATH SOLUBLE DITHIOCARBAMATE HAVING A RADICAL FROM THE GROUP CONSISTING OF 